1. Field of the Invention
The present invention relates to the technical field of objective lenses for use in optical pickup devices with which optical discs having different conditions on parameters such as substrate thickness and recording density are compatibly used in reproducing or recording information. The invention also relates to the technical field of the optical pickup devices.
2. Description of the Related Art
Optical recording and reproducing apparatus include optical disc players that read recorded information from optical discs used as optical information recording media. Exemplary optical discs include a LD (laser disc), a CD (compact disc) and a DVD (digital video disc or digital versatile disc). A compatible disc player can read information from a plurality of kinds of optical discs. Optical pickups used in a compatible disc player have an optical system that allows a light beam to illuminate the optical disc and which reads the return light from the optical disc.
Different optical discs are designed to have different specifications in various factors such as numerical aperture (NA), substrate thickness and optimal wavelength for reading light. Therefore, in order to realize an optical pickup suitable for use in a CD/DVD compatible player, the differences in at least two factors, i.e., numerical aperture (NA) and substrate thickness, must be corrected. The numerical aperture is 0.45 for the CD and 0.6 for the DVD whereas the substrate thickness is 1.2 mm for CDs and 0.6 mm for the DVD.
Consider the case where an objective lens primarily designed for the DVD is applied as a common lens which reproduces information from both the DVD and the CD using a light source with a wavelength of 650 nm for the DVD and a light source of 780 nm for the CD. The optical path in the case of reproducing information from the DVD is as shown in FIG. 34A and the aberration that occurs on the recording surface is negligible as shown in FIG. 35A. On the other hand, the optical path in the case of reproducing information from the CD is as shown in FIG. 34B and the aberration as shown in FIG. 35B occurs on the recording surface. This aberration is due to the difference in substrate thickness. In FIG. 34B, rays of incident light that pass through the marginal portion of the objective lens are greatly defocused, so as to produce only small effects on the characteristics of information reproduction from the CD. Hence, FIG. 34B shows only the optical path within the region of numerical aperture (NA) that affects the characteristics of information reproduction from the CD.
The vertical axis of the graphs shown in FIGS. 35A and 35B represents the aberration normalized by wavelength (to make 650 nm equal to one in FIG. 35A and 780 nm equal to one in FIG. 35B) whereas the horizontal axis represents the diameter of the objective lens (i.e., radial position from the optical axis).
In order to correct the aberration, two methods are currently used, one of which depends on a bifocal objective lens using a hologram and the other depends on two objective lenses having different focal lengths which are selectively operated for reading information from the DVD or the CD.
However, the bifocal objective lens using a hologram is difficult to manufacture and subject to a loss in the quantity of light. The selective use of two objective lenses requires a bulky pickup and incurs a substantial cost.